1. Technical Field
The present disclosure relates to a microelectromechanical device having an electromagnetic micromotor, in particular to a mass storage device made by probe-storage technology.
2. Description of the Related Art
As is known, current data-storage systems that exploit a technology based upon magnetism, such as, for example, computer hard disks, suffer from important limitations as regards the increase in the data-storage capacity and read/write rate and the reduction in their dimensions.
In the last few years, alternative data-storage systems have consequently been proposed, based upon the so-called “probe storage” technology, which exploits techniques of silicon micromachining, in order to reach data-storage densities and read/write rates that cannot be achieved with traditional techniques.
For example, the data-storage system proposed by IBM and referred to as “Millipede” (see in this connection “The “Millipede”-Nanotechnology Entering Data storage”, P. Vettiger et al., IEEE Transaction on nanotechnology, Vol. 1, No. 1, March 2002) exploits a type of technology based upon silicon-made, nanometric read/write heads, similar to the one exploited in atomic-force microscopes (AFMs) or in scanning tunnelling microscopes (STMs) to obtain atomic-scale images. For a better understanding, reference may be made to FIG. 1, which shows a general scheme of the “Millipede” system.
As is shown in the figure, a mass storage device 1 according to the “Millipede” system includes a two-dimensional array 2 of cantilever elements 3, made of silicon, obtained by exploiting the micromachining techniques and fixed to a common substrate 4, also made of silicon. Each of the cantilever elements 3 (usually referred to as “cantilevers”) functions as support for a respective read/write (R/W) head 6 made in an area corresponding to the end of the respective cantilever 3.
Each individual R/W head 6 can be controlled in reading or writing via an addressing technique similar to the one commonly used in DRAMs, and hence via two multiplexers 10, 11, that select, respectively, the rows and the columns of the two-dimensional array 2.
Present underneath the two-dimensional array 2 is a polymeric film 5 of the thickness of some tens of nanometers, made, for example, of polymethylmethacrylate (PMMA), and having the function of data-storage material. The polymeric film 5 is positioned on a movable platform 12 moved in the (mutually perpendicular) directions x, y via an actuation device (not shown in FIG. 1), including coils and miniaturized permanent magnets coupled so as to form electromagnetic micromotors.
Each R/W head 6 acts within a restricted data-storage area of its own, typically of the order of 100 μm2, so that, for example, in a 32×32 array, 1024 R/W heads 6 are present.
Each individual cantilever 3 performs data storage via the corresponding R/W head 6, by means of formation, in the polymeric film 5, of indentations 14 (shown only schematically in FIG. 1) having widths of, and being space apart by, some tens of nanometers.
The presence or absence of an indentation 14 encodes a datum to be stored in a binary format (for example, the presence of an indentation can represent a “1”, whilst the absence of an indentation can represent a “0”).
During writing, the indentations 14 are created by applying a local force on the polymeric film 5 through the R/W heads 6 and at the same time by heating locally the same polymeric film 5 to a high temperature (approximately 400° C.). Heating is obtained with a heater element of the resistive or junction type, set in an area corresponding to the R/W head 6 and traversed by electric current. When the R/W head 6 has reached the desired temperature, it is set in contact with the polymeric film 5, which is softened locally by the heat; consequently, the R/W head 6 penetrates within the polymeric film 5, generating the indentation 14.
Reading is carried out using the same heater element as temperature sensor, exploiting the variation of its current-voltage characteristic as a function of the temperature.
The actuation device is an important element in mass storage devices using probe-storage technology in so far as it has to enable positioning of the polymeric film with respect to the cantilevers in an extremely precise way. An inaccurate positioning, in fact, would nullify the advantages deriving from the extremely reduced dimensions of the R/W heads and would not enable the data-storage densities that are theoretically possible to be reached.
FIG. 2 shows a portion of an actuation device 50 of a known type for position control in the direction x. In particular, FIG. 2 illustrates a micromotor 51 connected to the movable platform 12 by an actuation bar 52, which is hinged to a base plate 53 by a fulcrum 55. The micromotor 21 includes a coil 56, fixed to the base plate 53, and two magnets 57, aligned in the direction x and opposite with respect to the coil 56. In addition, both of the magnets 57 are arranged so as to have the same pole facing the coil 56 (pole N in FIG. 2). The magnets 57 are fixed to a supporting frame 58, which is movable with respect to the base plate 53 and to the coil 56. The frame 58 is suspended above the supporting body 53 and is connected to one end of the actuation bar 52. Normally, the movable platform 12, the actuation bar 52, and the frame 58 are made from one and the same semiconductor wafer by micromachining techniques and form a single body.
By controlling the intensity and direction of a current circulating in the coil 56, it is possible to translate the magnets 57 and the frame 58 in the direction x, to obtain an opposite translation of the movable platform 12.
A similar structure, with micromotor and actuation bar (not shown herein) rotated through 90°, is used to translate the movable platform in the direction y.
The micromotor 51 enables a very accurate positioning of the movable platform but is far from efficient on account of the insufficient coupling between the magnets 57 and the coil 56. To obtain a force sufficient to control the movements of the movable platform, it is hence necessary to use magnets 57 of considerable dimensions or supply currents of high intensity for the coil 56. In the first case, the micromotor 51 is cumbersome and occupies an important percentage of the area of the mass storage device 1. In the second case, the consumption levels are sacrificed and worsen, a fact that is in any case disadvantageous.
Also assembly of the micromotor 1 presents difficulties because the magnets 57 must be separately bonded to the frame 58 and aligned to the coil 56. The probability of producing defective pieces is not negligible and adversely affects the production yield and cost.